1. Field of the Invention
This invention relates to tool-change apparatuses for a machining center and the like.
2. Prior Art
A representative example of the prior art tool change apparatuses is described in Japanese Laid-Open Patent Publication, unexamined, No. Sho-59-53136.
In this prior art apparatus, a spindle head slidably mounted on a column mounts the spindle rotatably. The spindle is formed with an axially extending operation bore, through which extends a draw bar arranged axially slidable and adapted for use in tool clamping purposes. By shifting the draw bar in one sliding direction, the tool inserted into and held in position is subjected to clamping, while, by shifting the draw bar in the opposite sliding direction, the said tool is unclamped. The tool-unclamping operation by the draw bar is always carried out, only after firm gripping of the tool by a pair or pairs of tool change arms mounted pivotably on the tool change shaft.
Another prior art apparatus is disclosed in Japanese Laid-Open Patent Publication, unexamined, No. Sho-55-58941. In this prior art arrangement, a tool change shaft, having change arms mounted therein, is arranged on a column or spindle head in such a way that after moving of the spindle head to a predetermined tool change station, the change shaft is rotated and the change arms are operated so as to allow the change arms to grip a spindle tool and a pot-held tool.
In a still another prior document, Japanese Laid-Open Patent Publication No. Sho-57-61441 is described an apparatus, wherein the column shiftably mounts a change shaft provided with two sets of mutually closable and openable change arms. After shifting of the spindle head to a predetermined change station, the change shaft is shifted and the change arms are operated to grip the spindle tool.
As stated above, in the prior art, tool change operation is carried out by unclamping the tools after the change arms have gripped the tools, withdrawing the tools by the arms, rotating the change shaft and then inserting new tools by the arms for subsequent clamping. Therefore, the clamping and unclamping operations are needed for tool change, which causes delay in the operation signals and a prolonged tool change time period.
Further, since the exchange shaft is rotated and shifted for gripping of tools after the spindle head has reached a predetermined tool change station, a time delay for generation and transfer of signals for related operational machine units and parts occurs during the rotary and shifting movements of the tool change arms, which gives rise to prolongation of time required for tool change.